The present invention relates to a mold additive suitable for use in a mold for continuous casting of steel and, more particularly, to a mold additive that contains synthetic calcium silicate as a base material and is suitable for use in a mold for continuous casting of steel.
Such a mold additive for use in a mold for continuous casting of steel is composed of a primary material such as portland cement, yellow phosphorus slag, or wollastonite and, as required, an SiO.sub.2 -containing material. The mold additive further includes a flux material such as soda ash, borax, cryolite, sodium fluoride or fluorite, and a carbonaceous material which serves as a melting-rate adjusting agent.
The mold additive is applied to the surface of a molten steel which has been poured into a mold, in which the mold additive is consumed while performing various functions. The important functions of the mold additive are (1) lubrication between the mold and a solidifying shell, (2) melting and absorption of any inclusion which rises to the surface of the molten steel, and (3) heat insulation with respect to the molten steel.
The progressive development of continuous casting processes in Japan is outstanding, and active efforts are being directed to increasing the proportion of HCR (hot charge rolling) or HDR (hot direct rolling) in a complete continuous casting process, the speeding up of casting, and so forth. In this situation, it is strongly desired to employ even stricter criterion for the mold additives which influence the quality of cast pieces and the stability of a casting process and, in addition, there is a demand for the supply of mold additives of various kinds which have characteristics that greatly differ from those of conventional mold additives. For these reasons, it has been proposed to provide a wide variety of chemical compositions which govern various characteristics of these mold additives, such as softening point, melting point, viscosity, surface tension and crystallization temperature. In particular, it is of great importance to adjust the weight ratio of CaO to SiO.sub.2 (hereinafter referred to as the "CaO/SiO.sub.2 ratio") as this ratio has a critical influence on these characteristics.
To achieve the functions (1) and (2) among the above noted functions of such a powder, it is most important to adjust the characteristics such as a softening point and viscosity of the mold additive which invites the importance of selection of chemical composition mentioned above. To achieve the function (3) regarding the heat insulation with respect to a molten steel, it is important to suitably select such powder characteristics as bulk density and spreadability, as well as the powder's melting rate which can be adjusted by a carbonaceous material.
The base material of the conventional mold additive is selected from among portland cement, yellow phosphorus slag, synthetic slag, wollastonite and the like. Each of these materials, however, has its merits and demerits, and there is no material having the characteristics to meet all the requirements of a base material.
For example, portland cement is characterized by its relatively stable chemical composition. Further, since its CaO/SiO.sub.2 ratio is higher than those of the other base materials, by combining the portland cement and a pearlite powder such as a light SiO.sub.2 -containing material, it is possible to achieve low bulk density and good heat insulation and to adjust the CaO/SiO.sub.2 ratio over a wide range. However, portland cement contains 4CaO.Al.sub.2 O.sub.3.Fe.sub.2 O.sub.3 in a range of 9 to 15% by weight and hence the resulting mold additive usually contains Fe.sub.2 O.sub.3 in an amount of about 2% by weight. The Fe.sub.2 O.sub.3 in the powder may react with a component (for example, Al) of a molten steel to cause contamination of the molten steel and this will at the same time result in a change in the characteristics of the mold additive. Accordingly, it is impossible to achieve stable lubrication. In addition, since a powder which includes portland cement as its base material is susceptible to hydration, it is difficult to subject such a powder to granulation using a generally adopted granulation process which comprises the steps of watering, kneading and extruding.
In contrast, yellow phosphorus slag, as well as synthetic slag having a composition analogous to that of the same, is a molten-quenched in water and crushed substance. Accordingly, such slag can be used as an amorphous material which excels in homogeneity of component distribution, and granulation thereof is easy. However, since the CaO/SiO.sub.2 ratio is relatively low (0.9-1.15), if a mold additive having a relatively high CaO/SiO.sub.2 ratio is to be manufactured, the amount of light SiO.sub.2 -containing material added needs to be reduced, thus resulting in the problem that the bulk density of the obtained powder is high. Another disadvantage of this material is that a powder composition having a CaO/SiO.sub.2 ratio of 1.15 or more cannot be obtained.
Wollastonite has an even lower CaO/SiO.sub.2 ratio and its use is therefore limited to an extremely narrow range of applications. In addition, wollastonite is inferior in terms of the stability of its components.
Various other methods have been proposed, for example, a method of producing a powder having a high CaO/SiO.sub.2 ratio by adding limestone or fluorite to a base material such as yellow phosphorus slag or wollastonite having a relatively low CaO/SiO.sub.2 ratio. However, all of the conventional methods involve problems with respect to the stability of the product quality. Accordingly, it has been impossible to provide a powder of desired quality.